Phosphate rock, which is mined for use in making fertilizer, often contains recoverable quantities of uranium. The phosphate rock is digested with sulfuric acid to form a phosphoric acid liquor. The uranium is oxidized to the +6 oxidation state and is extracted into an organic solvent.
If nitric acid is used as an oxidant, however, nitrates and nitrites are formed which enter the organic solvent with the uranium. If the uranium is stripped from the organic solvent with an aqueous solution containing ferrous ions, some of the ferrous ions will be used up reducing the nitrates and nitrites. This means that a higher concentration of ferrous ion is required to strip the uranium. At the resulting higher concentrations of ferric ion an iron complex, FeRH.sub.8 (PO.sub.4).sub.6, can precipitate which fills the stripper and must be periodically removed. (In the iron complex R is usually potassium, but may also be another alkali metal or ammonium.)